1. Field of Invention
This invention relates to defined competitive exclusion cultures or probiotics for the control of Salmonella colonization in swine.
Despite the efforts of researchers and public health agencies, the incidence of human salmonellosis has increased over the past 20 years. The number of actual reported cases of human Salmonella infection exceeds 40,000 per year. However, the Communicable Disease Center estimates that the true incidence of human Salmonella infections in the U.S. each year may be as high as 2 to 4 million. Animal food products, including swine, remain a significant source of human infection.
In addition to the impact of Salmonella on human health, Salmonella infections in swine cost the United States swine industry more than 100 million dollars annually (Schwartz, 1990, "Salmonellosis in Midwestern Swine", In: Proceedings of the United States Animal Health Assoc., pp. 443-449). In the U.S., salmonellosis caused by S. cholerasuis, the etiologic agent of swine parathyphoid, occurs most frequently. While pigs can be exposed to a broad host range of salmonellae, such as S. typhimurium, from a variety of sources, S. cholerasuis is a host adapted pathogen rarely isolated from non-swine sources (Schwartz, ibid). Thus, natural infection of new animals by S. cholerasuis occurs primarily via horizontal transmission from infected animals which shed the pathogens from their gastrointestinal tract.
2. Description of the Prior Art
Considering the widespread presence of Salmonella in the environment, it is unlikely that animals can be completely protected from Salmonella exposure. Therefore, researchers have continued to investigate means of increasing resistance to colonization in animals exposed to Salmonella. Studies have focused on the evaluation of vaccines, establishment of protective normal intestinal flora, and the identification of feed additives that will inhibit Salmonella growth and colonization. The role of host immunity against Salmonella colonization is unclear, and it also remains uncertain if stimulation of immune responses will effectively enhance colonization resistance. Experimental vaccines have not proven to be consistently effective.
It is well documented that the development of a normal intestinal microflora can increase resistance against Salmonella colonization of the gastrointestinal tract. In the poultry industry, oral inoculation of young chicks with anaerobic bacterial cultures of microflora, also known as probiotics (defined as bacterial cultures which have a beneficial effect on the animal to which they are administered), prepared from the cecal contents or fecal droppings of mature chickens has proven to effectively reduce Salmonella colonization Snoeyenbos et al., Avian Dis., 23:904-913, (1979), Schneitz et al., Acta Pathol. Microbiol. Scand. Sect. B., 89:109-116, (1981), and Stavric et al., J. Food Prot., 48:778-782, (1985)!. Conversely, poultry rearing practices that prevent chicks from becoming colonized by these cecal anaerobes make the chicks more susceptible to Salmonella colonization Pivnick et al., J. Food Prot., 44:909-916, (1981)!. These probiotics may decrease Salmonella colonization by rapidly colonizing the intestinal tract of the young chicks (Pivnick et al., ibid), by competing for attachment sites on the intestinal wall (Snoeyenbos et al., ibid), or by producing bacteriostatic or bactericidal short-chained volatile fatty acids Barnes et al., J. Hyg. Camb., 82:263-283, (1979) and Am. J. Clin. Nutr., 33:2426-2433, (1980), Corrier et al., Avian Dis., 34:668-676, (1990) and Avian Dis., 34:617-625, (1990), and Hinton et al., Avian Dis., 34:626-633, (1990)! that inhibit the growth of enteropathogens.
Establishment of normal intestinal flora in day-old chicks using mixed cultures of micro-organisms has been widely used to control Salmonella colonization in several European countries. Yet, because of the undefined number and types of micro-organisms present in mixed cultures, the system has not been widely accepted in the United States. One drawback to the widespread use of this method has been the fact that the composition of the product cannot be standardized, and thus the product cannot be stored or produced on a large scale without changes in composition and effectiveness. Also, because the starting material is always the intestinal content of an adult fowl, the product may contain pathogenic viruses, bacteria, or parasites, which may be dangerous to the health of the chicks. Further still, the U.S. Food & Drug Administration has recently required that all undefined cultures must be approved.